Bolt stress indicator



Sept. 24, 1963 HARRISON 3,104,645

BOLT STRESS INDICATOR Filed March 22, 1961 2 Sheets-Sheet 1 INVENTORHENRY HARRISON ATTORNEYS P 4, 1963 H. HARRISON I 3,104,645

BOLT STRESS INDICATOR Filed March 22, 1961 2 Sheets-Sheet 2 FIG.6'.

INVENTOR HENRY HARRISON ATTORNEYS United States Patent 3,104,645 BOLTSTRESS INDECATOR Henry Harrison, Frost Creek Drive, Locust Valley, N.Y.Filed Mar. 22, 1961, Ser. No. 103,182 18 Claims. (Cl. 116-114) Thisinvention relates to stress indicating devices, and has particularreference to a new and improved device for indicating when excessivestresses are imparted to a mine roof supporting bolt.

In the preparation of a mine tunnel for mining operations it is a commonpractice to reinforce the formation of rock above the mine roof.Conventional practice calls for drilling holes in the mine roof, andinserting bolts in the order of six feet or more in length into theseholes. The bolts have an expansible anchor means on the end thereofinserted into the drilled hole, and are provided with a mounting platewhich abuts against the mine roof. The bolt head is rotated, causingexpansion of the anchor means against the interior of the drilled hole.Tension is imparted to the bolt by drawing up the bolt head in tightabutment with the lower surface of the mounting plate.

While this practice is widely used to bind a rock formation above atunnel roof to inhibit the shifting of the rock, occasionally the rockformation will shift regardless of these bolts, causing grave danger topersonnel working in the mine tunnel. In this regard, such a shift inthe rock formation may result in a collapse of the mine tunnel roof. Itis, therefore, imperative to have a positive and reliable means forascertaining any shift in the rock formation above the tunnel roof.

When the rock formation above the tunnel roof shifts, a resulting stressis placed upon the bolts which are located within the rock, the tensilestress imparted thereto being transferred to a bolt head generallysecured to the end of the bolts which is visible from within the tunnel.Thus, by measuring this resulting increase of the stress imparted on themine roof supporting bolt, a reliable means for indicating dangerousshifts in rock formations can be realized. Such means for indicatingthis shifting of rock formation will, of course, only be as reliable asthe means for indicating the added stresses imparted to the bolt.

This problem has thus been appreciated in the prior art and numerousdevices of varying degrees of reliability have been developed. Suchdevices are generally afiixed to the lower end of the bolt and offer ameans for indicating adverse. stresses which may be imparted to thebolt. Such prior art devices are generally quite expensive tomanufactureand do not provide a totally reliable in dicating means which readilyindicates adverse bolt stresses without a very careful observation ofthe relative position of the various parts. A truly eflicient andpractical indicating means should provide a readily visible device whichclearly shows dangerous stresses imparted to the bolt, with no necessityfor careful measurement and observation.

.This invention is accordingly directed to an improved device forindicating when excessive stresses are imparted to mine roof bolts in asimple readily observable manner, and in turn, indicating when dangerousshifts in the rock formations above the mine tunnel roof have takenplace. Additionally, this invention approaches the manner of measuringthe added bolt stresses by a new and unusual means.

It is therefore an object of this invention to provide a bolt stressindicator which indicates excessive stress imparted to a bolt by anextrusion and resulting deflec-' tion of cantilevered flags disposedbetween the mine roof ceiling and the supporting bolt.

3,104,645 Patented Sept. 24, 1963 Another object of my invention is toprovide a bolt stress indicator which includes a means for extruding anddeflecting cantilevered indicating means, when excessive stresses areimparted to the bolt.

panying drawing, in which:

angle.

FIGURE 1 shows the assembled stress indicating device disposed in a rockformation above a mine tunnel roof;

FIGURE 2 is an enlarged elevation view of the assembly shown in FIGURE'1;

FIGURE 3 is a perspective view thereof with an electrical indicatingcircuit operable therewith;

FIGURE 4 is an enlarged fragmentary elevation view of the assembly inFIGURE 2, showing an extruded cantilevered flag; and,

FIGURE 5 is a modification of the assembly in FIG- URE 3, showing anadditional indicating means operable therewith.

FIGURE 6 is a plan view of still another modified embodiment of the flagaccording to this invention; and,

FIGURE 7 is an elevation section view of a modified embodiment of'theextruding surface shown in FIG- URE 6.

In accordance with the novel features of this invention, there isprovided ,a bearing extrusion member and cantilevered flags disposedthereon, resulting in an extrusion and downward deflection of these'flags when excessive stresses are imparted to a mine roof bolt.

Referring to the drawings, a mine roof 10 is shown having a longretaining bolt 12 disposed within drilled hole 14. The bolt is engagedwithin the rock formation by a conventional expansible anchoring shell16. Slidably mounted on the bolt 12 adjacent the roof 18 of the mine isa bolt stress indicator 20. The bolt stress indicator 20 is locatedbelow a mounting plate 22 which has an aperture 23 t-heretlirough, so asto be slidable on bolt 12 and abuts the ceiling 18 of the mine tunnel.Located beneath the mounting plate 22 and also slid on the bolt 12 is abearing shoe 24-. The bearing shoe 24 has bearing and extrusion members26 directed upwardly there from. Mounted between the mounting plate 22and the members 26 and the bearing shoe 24 are flags 28. The flags 28are mounted in a cantilevered fashion, being supported solely by members26. A self-aligning ballseat bearing 30 abuts against the lower surfaceof the bearing shoe 24. In this regard, mine roofs generally have arough, irregular surface, thus possibly resulting in poor alignment ofthe parts, unless such bearing aligning means are provided. The assemblyis held in engagement by :a conventional bolt head 32, which may be anupset head integral with the bolt or may be a threaded nut.

Referring now to FIGURES 2 and 4, the flags 28 are shown secured betweenmembers 26 and mounting plate 22. FIGURE 2 shows the flags mounted intheir normal assembled position, the flags extending radially outwardlyfrom the bolt 12 in a generally horizontal manner. .FIG- URE 4 shows theflags 2S extruded and deflected downwardly due to adverse stressesimparted to bolt 12.

One side ofthe extruding member 26 is cut to provide a curved extrudingsurface '34, while the other side of this extruding member isdrastically relieved along its inner surface 36 which intersects thesurface 34 at an Thus, the contour of the extruding surface as viewed insection is quite unsymmetric. Such a configuration tends to provideextrusion of the flag 28 in one direction only.

Various means can be provided to amplify the indication of excesivestress on the mine bolt, which results in the extrusion and downwarddeflection of the flags 28. For instance, FIGURE 3 shows an electricalcircuit 42 which is responsive to the downward deflection of the' flag28 has upturned side walls 52, and end wall 54.

The end wall 54 is preferably bent upwardly to a lesser degree than theside walls 52, forming an acute angle with the plane of flag 23. Aneasily fragible capsule 56 containing an odorous substance is placedthereon; and when the flag 28 is deflected downwardly, the capsule 56rolls off the flag 28' and breaks, causing an odorous gas to be spreadin the area. The particular configuration of the end wall '54 preventsthe capsule 56 from rolling off the flag 28 until the flag 28" has beendeflected downwardly through an appreciable angle.

Referring now to FIGURES 6 and 7, the flag 28 is shown to have opposedC-shaped slots 60 across the width thereof, the legs of the slot 60extending transversely of the lines of extrusion force P. The flag 28"in FIGURE 6 is also provided with cut-out areas 62 defining web portions64 adjacent the bolt 12. The web portions 64 buckle freely due to theextrusion force without noticeably resisting inward extrusion of theflag 28". The web portions 64- additionally facilitate arrangement offlag 28" with bolt 12 during assembly.

In FIGURE 7, the extruding surface 34 is shown to be symmetricallycurved while the flag 28" is shown partially extruded. As isappreciated, the contour of flag 28 adjacent the surface 34 becomesflatter as the thickness is decreased due to the extrusionforce.

Consideration will now be given to the extrusion of the normal tensionon the bolt results in a compressive stress on the flags 28. Thecapacity of the flags 28 to withstand extrusion can be controlled by theshape of the extruding surface. The amount of stress required to extrudea flag where a knife-edge extruding surface is used is considerably lessthan the stress required Where an arc-shaped extruding surface of arelatively large radius is utilized.

When a circular cylindrical extrusion surface 34 is provided on theextruding member 26, extrusion begins at a certain bearing force; and,if that forces is maintained the flag 28 is pinched down to a very smallthicknesstoo small to support the force of gravity on the cantileveredflag. During the extrusion process the thickness of extruding metaldecreases, so that an increased pressure is required to force it out.The width of hearing of the cylindrical surface against the flag also decreases, however, as the thickness decreases, so that the total bearingforce required to extrude the flag is substantially constant. The amountof bearing force required to cause extrusion is generally proportionalto the radius of curvature of the cylindrical surface of the extrudingmember.

When the extruding member 26 has an obtuse or acute angled edge pressinginto the flag, the bearing force necessary to cause extrusion diminishesrapidly as the thickness of metal decreases. On the other hand, when ithas a parallel flat bearing surface, the bearing force increases rapidlywith reduction of the squeezed metal thick- Q ness. Between these twoextremes, the surface 34 of the extruding member 26 can be shaped togive any desired intermediate relation between bearing force andthickness of squeezed-out metal. V

The force at which extrusion takes place depends primarily on threefactors: It is, as already noted, directly proportional to the radius ofcurvature of the extruding member; it is directly proportional to thewidth of the flag; and it is approximately proportional to the maximumyield strength of the material, in shear. For example, a flag ofSAE 3003aluminum alloy, pressed by extruding members having a radius ofcurvature of A; inch, isextruded by a force of about 16,000 pounds. Alowcarbon steel flag of the same size, under the same conditions, supportsabout 56,000 pounds. The thickness of the flag before assembly and itsinitial condition of ,work hardening do not affect the total force atwhich extrusion takes place.

As already stated, the compressive stress in the flag which must bewithstood by the parts between which it is squeezed, increases as thethickness decreases, and may reach very high values. The length,thickness, and density of a flag determine the bending moment, and thusdetermine how thin the flag must be squeezed before it will sag,indicating that extrusion has taken place. aluminum inch thick and teninches long, under the condition of the example in the above paragraph,will support itself horizontally until squeezed to less than .003 inchthickness. This corresponds to compressive stress of about 100,000p.s.i. A flag of low carbon steel under similar geometrical conditionswould have an internal compressive stress of over 300,000 p.s.i.Referring now to FIGURES 6 and 7, the modified embodiment of theinvention which reduces the internal compressive rower -supporting widthfails in bending at a greater thickness and a correspondingly smallerinternal cornpressive stress. However, it has been found that when thebearing shoe 24- and the mounting plate 22 are of hardened steel, theywill extrude even low carbon steel flags Without this refinement enough.to produce a clear,

indication.

At each of the extrusion members 26, the flag 28 is extruded bothinwardly toward the bolt 12 and outwardly away from the bolt 12. InFIGURE 6, the flag 28 may be slipped over the bolt for centering, inassembly, and yet the remaining portion of the flag around theboltbuckles freely, ofifering negligible resistance to inward extrusion ofthe flag material when excessive stress occurs. Unless such, provisionfor free buckling is made, resistance to inward extrusion may cause asubstantial variation in the bolt force at which extrusion takes place.

Friction at the boundary of the flagmaterial may also cause a variationin extruding force in the order of il0%. To control this variable, andalso to combat corrosion in the Warm, humid conditions frequently foundin mines, the parts of this device are thickly coated with grease at thetime of assembly.

Since the extruding operation moves the flag material outward at thesame time its thickness is reduced, the section of the flag becomes moreor less wedge shaped,

with the thinnest portion of the wedge where the extruding surface 34 isclosest to the mounting plate 22. Thus, the thicker portion of the wedgelimits the free deflection of the flag. To reduce this interference withdownward deflection, and thus reduce the amount of stress and extrusionrequired for deflection of the flags to thereby give a clear indicationthat extrusion has occurred, the bear-'- ing shoe 24 is preferably cutoff beyond a certain width as shown in the downwardly extending outsidewall 40 (FIGURE 4). The thickness of the flag 28 before extrusion takesplace, which determines the bending strength, is equal to the height ofthe sector of cylindrical surface 34 bearing against it.

An explanation will now be given of the assembly and operation of thebolt stress indicator 20. The self-aligning bearing is first slid on therod 12, followed by the bearing shoe, 24, the flag strip(s) 28, and themounting plate 22. The anchoring shell 16 is next assembled on the endof the rod 12. After an appropriate hole 14 has been drilled in theformation above the mine tunnel roof, a perpendicular spotface surfacemay be bored at the open end of the hole. Then the rod 12 is properlyinserted in the hole, with the mounting plate pressed up against thesurface of the mine roof. The anchoring shell 16 is expanded by rotationof the bolt head 32, which also results in tensioning the bolt to thedesired initial tension. During this initial tightening, the extrudingsurfaces 34 sink slightly into the flags, until the flags support theload. This results in a small downward deflection of the flags which maybe used to indicate when the bolt has sufficient initial tension. Whenthe bolt has been properly tightened, the flags may be bent backmanually to a horizontal position.

If and when the bolt 12 is subjected to adverse stress resulting from ashift of the rock formation, the stress will be imparted to the flagscausing extrusion and a downward deflection of the flags 28. It has beenfound, additionally, that the flags may be extruded to such an extentthat they actually break off.

' Thus, merely by observing the position of the flags 28, even anunskilled person will know if a rock formation shift above the tunnelroof has taken place.

Fabrication of the elements of this indicating device is economical. Thebearing shoe 24 is preferably produced as a rolled or drawn section,punched, sheared to length, and subsequently heat treated. Theself-aligning bearing cup is preferably produced by forging.

While the preferred embodiment of my invention has been shown, it isreadily apparent that considerable modi fication and additions may bemade thereto without departing from the scope of the invention.

It may be seen from the description and drawing that there is providedan invention which successfully achieves the objects and advantagesstated herein. Therefore, it is intended that this description beconsidered as illustrative only when consideration is given to theclaims.

What is claimed is:

1. In a mine roof bolt assembly having an elongated bolt with anexpansible anchoring means at its one end adapted for insertion in anupwardly extending hole in a mine roof with the anchoring meansend-foremost, the other end of said bolt having head means thereon, amounting plate having a roof-engaging surface and an aperturetherethrough for receiving said bolt, a stress indicator comprising:first means operatively carried by said bolt responsive to adversestress on said bolt for indicating when excessive stress is imparted tothe bolt; and a second means positioned adjacent said first means forreducing the cross-sectional area of said first means thereby providingthe indication of excessive bolt stress when the cross sectional area ofsaid first means is so reduced that said first means is deflecteddownward appreciably.

2. The device as defined in claim 1 wherein said first means includes atleast one defiectable flag extending radially outward of the bolt incantilevered fashion.

3. The device as defined in claim 2, further including a frangiblecapsule disposed on the upper surface of said flag, said capsule fallingfrom said flag, breaking and emitting an odorous gas upon appreciabledownward deflection of said flag.

4. The device as defined in claim 1, further including electricalindicating means responsive to appreciable downward deflection of saidfirst means to provide additional indication of the excessive stress onsaid bolt.

5. In a mine roof bolt assembly having an elongated thereof insertablein an upwardly extending hole in a mine roof, anchoring means endforemost and a mounting plate having a roof-engaging surface and acentral aperture therethrough .for receiving said bolt, and head meanson the other end of said bolt, a stress indicator comprising: at leastone cantilevered flag mountable below the mounting plate; means forextruding said flag disposed adjacent thereto, said means beingresponsive to excessive stress imparted to the bolt to cause a reductionin cross-sectional area and downward deflection of said flag.

6. The device as defined in claim 5, said means for extruding said flagdisposed on the bolt below said flag and above the head means of thebolt.

7. The device as defined in claim 6, further including a means foraligning said extruding means between said flag and the head means ofthe bolt.

8. The device as defined in claim 5, said means for extruding said flagincluding a curved surface adjacent to at least one surface of saidflag.

9. In a mine roof bolt assembly having an elongated bolt with anexpansible anchoring means on one end thereof insertable in an upwardlyextending hole in a mine roof, anchoring means end foremost and amounting plate having a roof-engaging surface and a central aperturetherefor receiving said bolt, and head means on the other end of saidbolt, a stress indicator comprising: a plurality of flags mountablebelow the mounting plate in symmetric cantilevered fashion; a bearingshoe slidably disposed on the bolt adjacent thereto; said bearing shoeincluding an extruding means extending therefrom in abutment with saidflags, said extruding means having linear flag-engaging surfacesdisposed in symmetric relationship with respect to the bolt axis, saidextruding means operable in response to excessive stress imparted to thebolt to reducing the cross-sectional area of said flags at a localizedarea thereof spaced from its free end, resulting in the downwarddeflection of said flag.

10. The device as described in claim 9, said bearing shoe disposed belowsaid flags and above the head means of the bolt.

11. The device as described in claim 10, further including aself-aligning bearing slidable on the bolt between said bearing shoe andthe bolt head means.

12. The device as defined in claim 9, further including a frangiblecapsule disposed on the upper surface of at least one of said flags,said capsule emitting an odorous gas upon extrusion and downwarddeflection of said flag.

13. The device as defined in claim 9, further including an electricalindicating means responsive to the extrusion and downward deflection ofat least one of said flags.

14. The device as described in claim 9 wherein each of said extrudingsurfaces includes a downwardly and outwardly curved surface terminatingat its inner extent with an inner surface transversely intersecting theinner extent of said outer surface wherein said extruding surface, insection, is unsymmetric and operative to provide, upon movement saidsurface toward its respective flag, an outward extrusion for reductionof the crosssection of said flag.

15. A device as defined in claim 9 wherein said plurality of flags isdefined by an elongated generally rectangular plate having a centralaperture for receiving said bolt, the opposed extents of said platedefining said flags.

16. A device as defined in claim 15 wherein said plate further includesopposed grooves therethrough adjacent said bolt defining web portionswhereby the material of said plate in the region adjacent of said boltis of a reduced cross-sectional area.

17. In a mine roof bolt assembly having an elongated bolt with anexpansible anchoring means at its one end adapted for insertion in anupwardly extending hole in a mine roof with the anchoring meansend-foremost, the other end of said bolt having head means thereon, a

mounting plate having a roof-engaging surface and an aperturetherethrough for receiving said bolt, a stress indicator comprising:first means operatively carried by said bolt adjacent said mountingplate and responsive to adverse stress on said bolt for indicating whenexcessive stress is imparted to the bolt; and a second means positionedbetween said first means and said bolt head means in engagement whensaid first means for extruding said first means and thereby reducing thecross-sectional area to a degree such that said first means is deflecteddownward appreciably to provide an indication ofthe excessive stress inresponse thereto.

18. The device as defined in claim 17 wherein the configuration of thesurface of said second means is operative 5 vide a visual indication ofsaid adverse stress.

References Cited in the file of this patent UNITED STATES PATENTS2,850,937 Ralston Sept. 9, 1958 Baynes Sept. 2, 1930

1. IN A MINE ROOF BOLT ASSEMBLY HAVING AN ELONGATED BOLT WITH ANEXPANSIBLE ANCHORING MEANS AT ITS ONE END ADAPTED FOR INSERTION IN ANUPWARDLY EXTENDING HOLE IN A MINE ROOF WITH THE ANCHORING MEANSEND-FOREMOST, THE OTHER END OF SAID BOLT HAVING HEAD MEANS THEREON, AMOUNTING PLATE HAVING A ROOF-ENGAGING SURFACE AND AN APERTURETHERETHROUGH FOR RECEIVING SAID BOLT, A STRESS INDICATOR COMPRISING:FIRST MEANS OPERATIVELY CARRIED BY SAID BOLT RESPONSIVE TO ADVERSESTRESS ON SAID BOLT FOR INDICATING WHEN EXCESSIVE STRESS IS IMPARTED TOTHE BOLT; AND A SECOND MEANS POSITIONED ADJACENT SAID FIRST MEANS FORREDUCING THE CROSS-SECTIONAL AREA OF SAID FIRST MEANS THEREBY PROVIDINGTHE INDICATION OF EXCESSIVE BOLT STRESS WHEN THE CROSS SECTIONAL AREA OFSAID FIRST MEANS IS SO REDUCED THAT SAID FIRST MEANS IS DEFLECTEDDOWNWARD APPRECIABLY.